Contra-rotating axial flow fan unit

ABSTRACT

An axial flow fan unit includes an intake side axial flow fan, an exhaust side axial flow fan, and at least one intermediate axial flow fan. Each of the axial flow fans includes an impeller having blades rotatable about a central rotation axis, a motor arranged to rotate the intake side impeller, and a housing having an inner circumferential surface that surrounds the impeller. The rotational direction of the impeller in the intermediate axial flow fan is different from that of the impellers adjacent thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a contra-rotating axial flow fan unit.

2. Description of the Related Art

Conventionally, a cooling fan is installed in electronic devices such asa personal computer, a server and the like to cool electronic partsenclosed within a casing thereof. The electronic parts are denselyarranged within the casing and, consequently, the heat emitted from theelectronic parts tends to stay within the casing. This requiresimprovement in the performance of the cooling fan. In particular, acooling fan capable of delivering an air under a high static pressureand with an increased volume is required in a large-sized electronicdevice such as a server or the like.

As a cooling fan for increasing the static pressure of the dischargedair, there is known a serially arranged axial flow fan unit in which twoaxial flow fans are connected in series. In the serially arranged axialflow fan unit, an air stream discharged from an upstream fan isintroduced into a downstream fan. Due to this construction, the seriallyarranged axial flow fan unit is capable of efficiently delivering airunder a high static pressure and with an increased volume.

In the serially arranged axial flow fan unit, however, the volume andthe static pressure of the discharged air are not increased by merelyconnecting two axial flow fans in series. Since the number of fans isincreased in the serially arranged axial flow fan unit, the amount ofthe electric current needed to rotate the impellers is also increased ascompared to a unitary axial flow fan. For that reason, there is a needto efficiently rotate the impellers of the serially arranged axial flowfan unit, thereby reducing the amount of the supplied current.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, a preferredembodiment of the present invention includes a contra-rotating axialflow fan unit having an intake side axial flow fan and an exhaust sideaxial flow fan. The intake side axial flow fan includes an intake sideimpeller having intake side blades rotatable about a central rotationaxis, an intake side motor arranged to rotate the intake side impeller,and an intake side housing having an inner circumferential surface thatsurrounds the intake side impeller. The exhaust side axial flow fanincludes an exhaust side impeller having exhaust side blades rotatableabout the central rotation axis, an exhaust side motor arranged torotate the exhaust side impeller, and an exhaust side housing having aninner circumferential surface that surrounds the exhaust side impeller.Further, at least one intermediate axial flow fan is arranged betweenthe intake side axial flow fan and the exhaust side axial flow fan.There may be a plurality (two or more) intermediate axial flow fans.

The intermediate axial flow fan includes an intermediate impeller havingintermediate blades rotatable about the central rotation axis, anintermediate motor arranged to rotate the intermediate impeller, and anintermediate housing having an inner circumferential surface thatsurrounds the intermediate impeller. A rotational direction of theintermediate impeller is different from that of the impellers adjacentthereto. Therefore, air is drawn into the contra-rotating axial flow fanunit through the intake side axial flow fan and is discharged to theoutside through the exhaust side axial flow fan. This makes it possibleto increase the volume and the static pressure of the air drawn thereinand discharged therefrom.

The intake side axial flow fan may include intake side support ribsarranged to interconnect the intake side motor and the intake sidehousing, the exhaust side axial flow fan may include a plurality ofexhaust side support ribs arranged to interconnect the exhaust sidemotor and the exhaust side housing, and the intermediate axial flow fanmay include intermediate support ribs arranged to interconnect theintermediate motor and the intermediate housing. Due to this feature, aportion of the swirling-direction velocity component of an air streamimpinges against the respective support ribs and is changed to an axialvelocity component. This increases the static pressure of the air.

It is preferable that the intake side support ribs are arranged betweenthe intake side impeller and the intermediate impeller, the intermediatesupport ribs are arranged between the intermediate impeller and anotherintermediate impeller or between the intermediate impeller and theexhaust side impeller, and the exhaust side support ribs are opposite tothe intermediate support ribs with the intermediate impeller lyingtherebetween.

In the above construction, it is preferable that the number of therespective blades differs from the number of the respective support ribswithin a respective axial fan. Due to this feature, the frequency of awind noise generated by the rotation of the respective impellers becomesdifferent from the frequency of an interfering noise generated when theair stream impinges against the respective support ribs, therebypreventing sympathetic vibration of the wind noise and the interferingnoise. As a result, it is possible to reduce the noise generated in thecontra-rotating axial flow fan unit.

In a preferred embodiment of the present invention, the intermediatehousing may be axially opposite to the intake side housing, the exhaustside housing, or another intermediate housing. The respective housingsmay make contact with one another or may be opposite to one another viagaps therebetween. The respective axial flow fans may be fixed to oneanother by screws, engaging structures or other suitable fixingmechanisms or materials.

It is preferable that the intake side support ribs, the exhaust sidesupport ribs, and the intermediate support ribs are substantially flat.Each of the intake side support ribs, the exhaust side support ribs, andthe intermediate support ribs may have an upper edge and a lower edge.Each of the intake side support ribs, the exhaust side support ribs, andthe intermediate support ribs may have an air receiving surface definedbetween the upper edge and the lower edge to confront an air streamflowing from the intake side impeller toward the exhaust side impeller.It is preferable that the air receiving surface is inclined with respectto the central rotation axis to axially face toward an exhaust side.

It is preferable that the inclination of each of the blades relative tothe central rotation axis is substantially the same as the inclinationof each of the support ribs with respect to the central rotation axis.This ensures that the air drawn into the contra-rotating axial flow fanunit can smoothly flow through the vicinity of the respective blades andthe respective support ribs.

In the respective support ribs of the axial flow fans, it is preferredthat the upper edge of each of the support ribs is positioned upstreamof the lower edge thereof with reference to the rotational direction ofthe respective impellers. This allows the air stream to smoothly flowthrough the vicinity of the respective support ribs with a minimal lossof energy of the air stream.

The respective support ribs may be positioned in an intake side of eachof the housings. This ensures that a straightened air stream is smoothlydrawn into the contra-rotating axial flow fan unit thereby reducing thenoise generated in the contra-rotating axial flow fan unit.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a contra-rotating axial flow fanunit in accordance with a first preferred embodiment of the presentinvention.

FIG. 2 is a sectional view of the contra-rotating axial flow fan unit inaccordance with the first preferred embodiment of the present invention.

FIG. 3 is a sectional view illustrating some of the blades and ribsincluded in the contra-rotating axial flow fan unit in accordance withthe first preferred embodiment of the present invention.

FIG. 4 is a perspective view showing a modified example of thecontra-rotating axial flow fan unit in accordance with the firstpreferred embodiment of the present invention.

FIG. 5 is a sectional view of the modified example of thecontra-rotating axial flow fan unit in accordance with the firstpreferred embodiment of the present invention.

FIG. 6 is a perspective view showing a contra-rotating axial flow fanunit in accordance with a second preferred embodiment of the presentinvention.

FIG. 7 is a sectional view of the contra-rotating axial flow fan unit inaccordance with the second preferred embodiment of the presentinvention.

FIG. 8 is a sectional view illustrating some of the blades and ribsincluded in the contra-rotating axial flow fan unit in accordance withthe second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 through 8, preferred embodiments of the presentinvention will be described in detail. It should be noted that in theexplanation of the preferred embodiments of the present invention, whenpositional relationships among and orientations of the differentcomponents are described as being up/down or left/right, ultimatelypositional relationships and orientations that are in the drawings areindicated; positional relationships among and orientations of thecomponents once having been assembled into an actual device are notindicated. Meanwhile, in the following description, an axial directionindicates a direction parallel or substantially parallel to a rotationaxis, and a radial direction indicates a direction perpendicular orsubstantially perpendicular to the rotation axis.

As shown in FIGS. 1 and 2, the contra-rotating axial flow fan unit 1 inaccordance with a first preferred embodiment of the present inventionpreferably is a triple contra-rotating axial flow fan including threeaxial flow fans, i.e., an intake side axial flow fan 2, a firstintermediate axial flow fan 4 and an exhaust side axial flow fan 3. Therespective axial flow fans are fixed to one another by screws, engagingstructures (not shown) or other suitable fixing mechanisms or materials.

As illustrated in FIG. 2, an intake side impeller 21, an intermediateimpeller 41, and an exhaust side impeller 31 are respectively arrangedinside the intake side axial flow fan 2, the first intermediate axialflow fan 4, and the exhaust side axial flow fan 3.

The intake side impeller 21 and the intermediate impeller 41 are rotatedabout a central rotation axis J1 in different directions. Likewise, theintermediate impeller 41 and the exhaust side impeller 31 are rotatedabout the central rotation axis J1 in different directions. In thepresent preferred embodiment, when seen from the axially upper side inFIG. 1, the intake side impeller 21 rotates clockwise, the intermediateimpeller 41 rotates counterclockwise, and the exhaust side impeller 31rotates clockwise. Consequently, air is drawn into the intake side axialflow fan 2 and delivered to the exhaust side axial flow fan 3. Thisgenerates an air stream flowing along the central rotation axis J1.

As can be seen in FIG. 2, the intake side axial flow fan 2 includes anintake side impeller 21, an intake side motor 22, an intake side housing23, and a plurality (for example, eight, in the present preferredembodiment) of intake side support ribs 24.

The intake side impeller 21 includes a plurality of (for example, seven,in the present preferred embodiment) intake side blades 211 and asubstantially cylindrical closed-top hub 212. On the outer surface ofthe hub 212, the intake side blades 211 extend radially outward aboutthe central rotation axis J1 and are circumferentially arrangedpreferably at an equal pitch. The hub 212 and the intake side blades 211are preferably made of a synthetic resin and are preferably integrallyformed by injection molding. When seen from the axially upper side inFIG. 2, the intake side impeller 21 is rotated clockwise about thecentral rotation axis J1 by the intake side motor 22. This generates anair stream axially flowing from the intake side blades 211.

The intake side housing 23 is a hollow member preferably made of asynthetic resin and has an inner circumferential surface having asubstantially cylindrical shape designed to surround the intake sideimpeller 21. Within the intake side housing 23 (namely, on the innercircumferential surface of the intake side housing 23), there is defineda flow path through which the air stream generated by the rotation ofthe intake side impeller 21 flows. The intake side housing 23 isprovided with an upper end portion and a lower end portion each havingan inner circumferential surface arranged so that the distance betweenthe central rotation axis J1 and the inner circumferential surface canbe increased axially upward or downward. This ensures that the air issmoothly drawn into and discharged from the intake side housing 23 asthe intake side blades 211 rotate.

The intake side support ribs 24 are preferably made of a synthetic resinand substantially flat. The intake side support ribs 24 are arrangedbelow the intake side impeller 21 (namely, between the intake sideimpeller 21 and the first intermediate axial flow fan 4) and extendradially outward from the intake side motor 22 so that they can beconnected to the intake side housing 23 to thereby support the intakeside motor 22.

As shown in FIG. 2, the intake side motor 22 includes a stator portion221 and a rotor portion 222. The rotor portion 222 is supported by abearing mechanism so that the rotor portion 222 can rotate relative tothe stator portion 221.

The stator portion 221 is provided with a base portion 2211 that issubstantially disk-shaped when seen in a plan view. The base portion2211 is fixed to the inner circumferential surface of the intake sidehousing 23 through the intake side support ribs 24 to thereby hold thestator portion 221 in place. The base portion 2211 is preferably made ofa synthetic resin and is preferably integrally formed with the intakeside support ribs 24 and the intake side housing 23 by injection-moldingthe synthetic resin. However, the material and the method used inproducing the base portion 2211, the intake side support ribs 24, andthe intake side housing 23 are not limited to synthetic resin andinjection molding. Alternatively, the base portion 2211, the intake sidesupport ribs 24 and the intake side housing 23 may be formed by, e.g.,die-casting aluminum with an aluminum alloy or other suitable materialsand/or processes.

As illustrated in FIG. 2, a substantially cylindrical bearing holderportion 2212 is fixed to the substantially central region of the baseportion 2211 so that it can protrude upward from the base portion 2211.The bearing holder portion 2212 is preferably made of a metal and ispreferably integrally fixed to the base portion 2211 byinjection-molding (e.g., insert-molding) a resin. The bearing holderportion 2212 may be either formed from a synthetic resin or integrallyformed with the base portion 2211 by injection-molding a resin ordie-casting aluminum. The material and the method used in producing thebearing holder portion 2212 are not particularly limited. Ball bearings2213 and 2214 are provided in the axial upper and lower regions insidethe bearing holder portion 2212, thus defining a portion of the bearingmechanism. An elastic member (e.g., a spring) is preferably arranged topre-compress the ball bearing 2214 from above.

The stator portion 221 is further provided with a stator 2215 and acircuit board 2216. The stator 2215 is attached to the outer surface ofthe bearing holder portion 2212. The circuit board 2216 has asubstantially annular flat shape and is attached to the lower side ofthe stator 2215. The circuit board 2216 is provided with a circuitarranged to control the rotation of the rotor portion 222 and iselectrically connected to the stator 2215 through a conductive pin (notshown) and the like. An electric current and a control signal are sentfrom an external power source (not shown) to the circuit board 2216 viaa lead line group (not shown) having a plurality of lead lines tiedtogether.

The rotor portion 222 includes a yoke 2221, a field magnet 2222, and ashaft 2223.

The yoke 2221 is preferably made of a magnetic metal and has asubstantially cylindrical closed-top shape. The field magnet 2222preferably has a substantially cylindrical shape and is fixed to theinner surface of the cylindrical portion of the yoke 2221 by an adhesiveagent or fixing materials or mechanisms. The field magnet 2222 isradially opposite to the stator 2215. An axially downward protrudingcylinder portion is provided in the substantially central region of thecover portion of the yoke 2221 and may be formed by burring or othersuitable process. The shaft 2223 is press-fitted into the cylinderportion in a coaxial relationship with the central rotation axis J1. Theshaft 2223 is inserted into the bearing holder portion 2212 and issupported by the ball bearings 2213 and 2214 for rotation relative tothe stator portion 221. That is, in the intake side axial flow fan 2,the shaft 2223 and the ball bearings 2213 and 2214 define a bearingmechanism arranged to support the yoke 2221 so that the yoke 2221 canrotate about the central rotation axis J1 with respect to the baseportion 2211.

As a driving current is supplied from the external power source to thestator 2215 through the lead line group and the circuit board 2216, thetorque acting about the central rotation axis J1 is generated betweenthe stator 2215 and the field magnet 2222. The rotor portion 222 and theintake side impeller 21 are rotated by the torque thus generated. Thedriving current supplied to the stator 2215 is controlled by the circuitin the circuit board 2216. This makes it possible to rotate the intakeside impeller 21 at a predetermined rotation speed.

The first intermediate axial flow fan 4 includes a first intermediateimpeller 41, a first intermediate motor 42, a first intermediate housing43, and a plurality of (for example, eight, in the present preferredembodiment) first intermediate support ribs 44.

The first intermediate impeller 41 includes first intermediate blades411 and a substantially cylindrical closed-top first hub 412. The firstintermediate blades 411 extend radially outward and arecircumferentially arranged at an equal pitch. The first intermediateblades 411 and the first hub 412 are all preferably made of a syntheticresin and integrally formed by injection molding.

The first intermediate motor 42 is arranged to rotate the firstintermediate impeller 41 counterclockwise about the central rotationaxis J1 when seen from the upper side in FIG. 2. This generates an airstream flowing along the central rotation axis J1.

The first intermediate housing 43 is a hollow member preferably made ofa synthetic resin and has an inner circumferential surface having asubstantially cylindrical shape surrounding the first intermediateimpeller 41. Within the first intermediate housing 43 (namely, on theinner circumferential surface of the first intermediate housing 43),there is defined a flow path through which the air stream generated bythe rotation of the intermediate impeller 41 flows. The firstintermediate housing 43 is provided with an upper end portion and alower end portion each having an inner circumferential surface arrangedso that the distance between the central rotation axis J1 and the innercircumferential surface can be increased axially upward or downward.This ensures that the air is smoothly drawn into and discharged from thefirst intermediate housing 43 as the first intermediate blades 411rotate.

The first intermediate support ribs 44 are preferably made of asynthetic resin and substantially flat. The first intermediate supportribs 44 are arranged below the intermediate impeller 41 (namely, betweenthe intermediate impeller 41 and the exhaust side axial flow fan 3) andextend radially outward from the first intermediate motor 42 so thatthey can be connected to the first intermediate housing 43 to therebysupport the first intermediate motor 42.

As shown in FIG. 2, the first intermediate motor 42 includes a statorportion 421 and a rotor portion 422. The rotor portion 422 is supportedby the below-mentioned bearing mechanism so that it can rotate relativeto the stator portion 421.

The stator portion 421 is provided with a base portion 4211 that issubstantially disk-shaped when seen in a plan view. The base portion4211 is fixed to the inner circumferential surface of the firstintermediate housing 43 through the first intermediate support ribs 44to thereby hold the stator portion 421 in place. The base portion 4211is preferably made of a synthetic resin and is preferably integrallyformed with the first intermediate support ribs 44 and the firstintermediate housing 43 by injection-molding the synthetic resin.However, the material and the method used in producing the base portion4211, the first intermediate support ribs 44, and the first intermediatehousing 43 are not limited to synthetic resin and injection molding.Alternatively, the base portion 4211, the first intermediate supportribs 44, and the first intermediate housing 43 may be formed by, e.g.,die-casting aluminum with an aluminum alloy or other suitable processesor materials.

As illustrated in FIG. 2, a substantially cylindrical bearing holderportion 4212 is fixed to the substantially central region of the baseportion 4211 so that it can protrude upward from the base portion 4211.The bearing holder portion 4212 is preferably made of a metal and ispreferably integrally fixed to the base portion 4211 byinjection-molding (e.g., insert-molding) with a resin. The bearingholder portion 4212 may be either formed from a synthetic resin,aluminum, or aluminum alloy or integrally formed with the base portion4211 and the first intermediate housing 43 by injection-molding a resinor die-casting aluminum. The material and the method used in producingthe bearing holder portion 4212 are not particularly limited. Ballbearings 4213 and 4214 are provided in the axial upper and lower regionsinside the bearing holder portion 4212, thus defining a portion of thebearing mechanism. An elastic member (e.g., a spring) is preferablyarranged to pre-compress the ball bearing 4214 from above.

The stator portion 421 is provided with a stator 4215 and a circuitboard 4216. The stator 4215 is attached to the outer surface of thebearing holder portion 4212. The circuit board 4216 has a substantiallyannular flat shape and is attached to the lower side of the stator 4215.The circuit board 4216 is provided with a circuit arranged to controlthe rotation of the rotor portion 422 and is electrically connected tothe stator 4215 through a conductive pin (not shown) and the like. Anelectric current and a control signal are sent from an external powersource (not shown) to the circuit board 4216 via a lead line group (notshown) having a plurality of lead lines tied together.

The rotor portion 422 includes a yoke 4221, a field magnet 4222, and ashaft 4223. The yoke 4221 has a substantially cylindrical closed-topshape and is preferably made of a magnetic metal. The field magnet 4222preferably has a substantially cylindrical shape and is fixed to theinner surface of the cylindrical portion of the yoke 4221 by an adhesiveagent or other suitable fixing materials or mechanisms. The field magnet4222 is radially opposite to the stator 4215. An axially downwardprotruding cylinder portion is provided in the substantially centralregion of the cover portion of the yoke 4221 and may be formed byburring or other suitable process. The shaft 4223 is press-fitted intothe cylinder portion in a coaxial relationship with the central rotationaxis J1.

The shaft 4223 is inserted into the bearing holder portion 4212 and issupported by the ball bearings 4213 and 4214 for rotation relative tothe stator portion 421. That is, in the first intermediate axial flowfan 4, the shaft 4223, and the ball bearings 4213 and 4214 define abearing mechanism arranged to support the yoke 4221 so that it canrotate about the central rotation axis J1 with respect to the baseportion 4211.

As a driving current is supplied from the external power source to thestator 4215 through the lead line group and the circuit board 4216, thetorque acting about the central rotation axis J1 is generated betweenthe stator 4215 and the field magnet 4222. The rotor portion 422 and thefirst intermediate impeller 41 are rotated by the torque thus generated.The driving current supplied to the stator 4215 is controlled by thecircuit in the circuit board 4216. This makes it possible to rotate thefirst intermediate impeller 41 at a predetermined rotation speed.

The exhaust side axial flow fan 3 includes an exhaust side impeller 31,an exhaust side motor 32, an exhaust side housing 33 and a plurality of(for example, eight, in the present preferred embodiment) exhaust sidesupport ribs 34.

The exhaust side impeller 31 includes a plurality of (for example,seven, in the present preferred embodiment) exhaust side blades 311 anda substantially cylindrical closed-top hub 312. On the outer surface ofthe hub 312, the exhaust side blades 311 extend radially outward and arecircumferentially arranged at an equal pitch. When seen from the axiallyupper side in FIG. 2, the exhaust side impeller 31 is rotated clockwiseabout the central rotation axis J1 by the exhaust side motor 32. Thisgenerates an air stream axially flowing from the exhaust side impeller31.

The exhaust side housing 33 is a hollow member preferably made of asynthetic resin and has an inner circumferential surface ofsubstantially cylindrical shape designed to surround the exhaust sideimpeller 31. Within the exhaust side housing 33 (namely, on the innercircumferential surface of the exhaust side housing 33), there isdefined a flow path through which the air stream generated by therotation of the exhaust side impeller 31 flows. The exhaust side housing33 is provided with an upper end portion and a lower end portion eachhaving an inner circumferential surface arranged so that the distancebetween the central rotation axis J1 and the inner circumferentialsurface can be increased axially upward or downward. This ensures thatthe air is smoothly drawn into and discharged from the exhaust sidehousing 33 as the exhaust side blades 311 rotate.

The exhaust side support ribs 34 are arranged below the exhaust sideimpeller 31 and extend radially outward from the exhaust side motor 32about the central rotation axis J1 so that they can be connected to theexhaust side housing 33 to thereby support the exhaust side motor 32.

As shown in FIG. 2, the exhaust side motor 32 includes a stator portion321 and a rotor portion 322. The rotor portion 322 is supported by abearing mechanism so that it can rotate about the central rotation axisJ1 relative to the stator portion 321.

The stator portion 321 is provided with a base portion 3211 that issubstantially disk-shaped when seen in a plan view. The base portion3211 is fixed to the inner circumferential surface of the exhaust sidehousing 33 through the exhaust side support ribs 34 to thereby hold thestator portion 321 in place. The base portion 3211 is preferably made ofa synthetic resin and is preferably integrally formed with the exhaustside support ribs 34 and the exhaust side housing 33 byinjection-molding the synthetic resin. However, the material and themethod used in producing the base portion 3211, the exhaust side supportribs 34, and the exhaust side housing 33 are not limited to syntheticresin and injection molding. Alternatively, the base portion 3211, theexhaust side support ribs 34, and the exhaust side housing 33 may beformed by, e.g., die-casting aluminum or other materials.

As illustrated in FIG. 2, a substantially cylindrical bearing holderportion 3212 is fixed to the substantially central region of the baseportion 3211 so that it can protrude upward from the base portion 3211.The bearing holder portion 3212 is preferably made of a metal and ispreferably integrally fixed to the base portion 3211 byinjection-molding (e.g., insert-molding) with a resin. The bearingholder portion 3212 may be either formed from a synthetic resin,aluminum, or aluminum alloy or integrally formed with the base portion3211 and the exhaust side housing 33 by injection-molding a resin ordie-casting aluminum. The material and the method used in producing thebearing holder portion 3212 are not particularly limited. Ball bearings3213 and 3214 are provided in the upper and lower regions inside thebearing holder portion 3212, thus defining a portion of the bearingmechanism. An elastic member (e.g., a spring) is preferably arranged topre-compress the ball bearing 3214 from above.

The stator portion 321 is provided with a stator 3215 and a circuitboard 3216. The stator 3215 is attached to the outer surface of thebearing holder portion 3212. The circuit board 3216 has a substantiallyannular flat shape and is attached to the lower side of the stator 3215.The circuit board 3216 is provided with a circuit arranged to controlthe rotation of the rotor portion 322 and is electrically connected tothe stator 3215 through a conductive pin and the like. An electriccurrent and a control signal are sent from an external power source (notshown) to the circuit board 3216 via a lead line group (not shown)having a plurality of lead lines tied together.

The rotor portion 322 includes a yoke 3221, a field magnet 3222, and ashaft 3223. The yoke 3221 has a substantially cylindrical closed-topshape and is preferably made of a magnetic metal. The field magnet 3222has a substantially cylindrical shape and is fixed to the inner surfaceof the cylindrical portion of the yoke 3221 by an adhesive agent orother suitable fixing materials or mechanisms. The field magnet 3222 isradially opposite to the stator 3215. An axially downward protrudingcylinder portion is provided in the substantially central region of thecover portion of the yoke 3221 and may preferably be formed by burringor other suitable process. The shaft 3223 is press-fitted into thecylinder portion in a coaxial relationship with the central rotationaxis J1.

The shaft 3223 is inserted into the bearing holder portion 3212 and issupported by the ball bearings 3213 and 3214 for rotation relative tothe stator portion 321. That is, in the exhaust side axial flow fan 3,the shaft 3223, and the ball bearings 3213 and 3214 define a bearingmechanism arranged to support the yoke 3221 so that it can rotate aboutthe central rotation axis J1 with respect to the base portion 3211.

As a driving current is supplied from the external power source to thestator 3215 through the lead line group and the circuit board 3216, thetorque acting about the central rotation axis J1 is generated betweenthe stator 3215 and the field magnet 3222. The rotor portion 322 and theintake side impeller 31 are rotated by the torque thus generated. Thedriving current supplied to the stator 3215 is controlled by the circuitin the circuit board 3216. This makes it possible to rotate the exhaustside impeller 31 at a predetermined rotation speed.

The upper and lower end surfaces of the first intermediate housing 43axially coincide with the lower end surface of the intake side housing23 and the upper end surface of the exhaust side housing 33,respectively. When observing the contra-rotating axial flow fan unit 1as a whole, the respective flow paths (i.e., the respective innercircumferential surfaces) of the intake side housing 23, the firstintermediate housing 43, and the exhaust side housing 33 aresuccessively and smoothly connected to one another along the axialdirection, thereby defining a single flow path (i.e., a single innercircumferential surface). As shown in FIG. 2, the intake side impeller21, the intake side support ribs 24, the first intermediate impeller 41,the first intermediate support ribs 44, the exhaust side impeller 31,and the exhaust side support ribs 34 are arranged in the single flowpath one below another from the upper side (i.e., the intake side) inFIG. 2.

The number of the intake side support ribs 24 differs from the number ofthe blades (the intake side blades 211 and the first intermediate blades411) of the impellers axially adjacent thereto (i.e., the intake sideimpeller 21 and the first intermediate impeller 41). Furthermore, thenumber of the first intermediate blades 411 differs from the number ofthe support ribs axially adjacent thereto (i.e., the intake side supportribs 24 and the first intermediate support ribs 44). Moreover, thenumber of the first intermediate support ribs 44 differs from the numberof the blades (i.e., the first intermediate blades 411 and the exhaustside blades 311) of the impellers axially adjacent thereto (i.e., thefirst intermediate impeller 41 and the exhaust side impeller 31). Inaddition, the number of the exhaust side blades 311 of the exhaust sideimpeller 31 differs from the number of the first intermediate supportribs 44 and the exhaust side support ribs 34. That is, in the case ofthe contra-rotating axial flow fan unit 1, the axially adjacent bladesand support ribs differ in number from one another.

The air stream generated by the rotation of the respective bladesimpinges on the respective support ribs axially adjacent to the blades,thus generating noise. If the number of the respective blades and therespective support blades are set as mentioned above, the frequency of awind noise generated by the rotation of the respective impellers doesnot coincide with the frequency of an interfering noise generated whenthe air stream impinges against the respective support ribs. Therefore,it is possible to prevent an increase in noise which would otherwiseoccur by the sympathetic vibration of the wind noise and the interferingnoise. This makes it possible to minimize the noise generated from thecontra-rotating axial flow fan unit 1.

As described above, a single flow path defined by the innercircumferential surfaces of the respective housings successivelyconnected to one another in the axial direction is formed within thecontra-rotating axial flow fan unit 1. The air stream introduced fromthe intake side axial flow fan 2 is allowed to smoothly flow along thesingle flow path (i.e., the inner circumferential surfaces of therespective housings). Then, the air stream is discharged from theexhaust side axial flow fan 3 to the outside with a minimal loss ofenergy due to frictional contact of the air stream with the flow path.

FIG. 3 is a sectional view illustrating only the cross-sections of theintake side blades 211, the intake side support ribs 24, the firstintermediate blades 411, the first intermediate support ribs 44, theexhaust side blades 311, and the exhaust side support ribs 34 takenalong a cylindrical plane having an arbitrary radius measured from thecentral rotation axis J1 in FIG. 2.

Each of the intake side support ribs 24 preferably is substantially flatand has an upper edge 241 positioned adjacent to each of the intake sideblades 211 and a lower edge 242 positioned adjacent to the intermediateaxial flow fan 4. The upper edge 241 is positioned upstream of the loweredge 242 with reference to the rotational direction R2 of each of theintake side blades 211. Each of the intake side support ribs 24 has anair receiving surface 243 inclined relative to the central rotation axisJ1 to face toward the exhaust side. Thus, the air receiving surface 243confronts the air stream generated by the rotation of the intake sideblades 211.

The air stream generated by the intake side impeller 21 can be dividedinto different directional components, including a radially outwardcentrifugal velocity component, a central rotation axis directionvelocity component acting parallel to the central rotation axis J1, anda swirling velocity component acting tangential to the rotation of theimpeller.

The swirling-direction velocity component acts substantially in the samedirection as the rotational direction R2. A portion of theswirling-direction velocity component impinges against each of theintake side support ribs 24 and the air receiving surface 243 thereofand, therefore, is changed to a velocity component acting in the samedirection as the central rotation axis J1. This increases the staticpressure of the air.

The air stream passed through the vicinity of the air receiving surface243 of each of the intake side support ribs 24 is introduced toward eachof the first intermediate blades 411 that rotates in the rotationaldirection R4. As shown in FIG. 3, each of the first intermediate blades411 is inclined with respect to the central rotation axis J1substantially in the same direction as is each of the intake sidesupport ribs 24. Therefore, the air stream whose velocity component hasbeen changed by the intake side support ribs 24 is smoothly introducedtoward the first intermediate blades 411, which makes it possible tominimize the reduction of energy of the air stream.

Each of the first intermediate support ribs 44 has an upper edge 441positioned adjacent to each of the first intermediate blades 411 and alower edge 442 positioned adjacent to the exhaust side axial flow fan 3.The upper edge 441 is positioned upstream of the lower edge 442 withreference to the rotational direction R4 of each of the firstintermediate blades 411. Each of the first intermediate support ribs 44has an air receiving surface 443 inclined relative to the centralrotation axis J1 to face toward the exhaust side. Thus, the airreceiving surface 443 confronts the air stream generated by the rotationof the first intermediate blades 411.

The swirling-direction velocity component generated by the rotation ofthe first intermediate blades 411 acts substantially in the samedirection as the rotational direction R4. A portion of theswirling-direction velocity component impinges against each of the firstintermediate support ribs 44 and, therefore, is changed to a velocitycomponent acting in the same direction as the central rotation axis J1.This increases the static pressure of the air.

The air stream passed through the vicinity of the air receiving surface443 is introduced toward each of the exhaust side blades 311. As shownin FIG. 3, each of the exhaust side blades 311 is inclined with respectto the central rotation axis J1 substantially in the same direction asis each of the first intermediate support ribs 44. Therefore, the airstream whose velocity component has been changed by the firstintermediate support ribs 44 is smoothly introduced toward the exhaustside blades 311. As a result, it is possible to minimize the reductionof energy of the air stream.

Each of the exhaust side support ribs 34 has an upper edge 341positioned adjacent to each of the exhaust side blades 311 and a loweredge 342 positioned adjacent to the exhaust side. The upper edge 341 ispositioned upstream of the lower edge 342 with reference to therotational direction R3 of each of the exhaust side blades 311. Each ofthe exhaust side support ribs 34 has an air receiving surface 343inclined relative to the central rotation axis J1 to face toward theexhaust side. Thus, the air receiving surface 343 confronts the airstream generated by the rotation of the exhaust side blades 311.

The swirling-direction velocity component generated by the rotation ofthe exhaust side blades 311 acts substantially in the same direction asthe rotational direction R3. A portion of the swirling-directionvelocity component impinges against each of the exhaust side supportribs 34 and, therefore, is changed to a velocity component acting in thesame direction as the central rotation axis J1. This increases thestatic pressure of the air.

The air stream passed through the air receiving surface 343 of each ofthe exhaust side support ribs 34 is discharged to the outside of theexhaust side housing 33 (namely, to the outside of the contra-rotatingaxial flow fan unit 1).

The construction described above ensures that the air drawn into thecontra-rotating axial flow fan unit 1 passes through the vicinity of therespective blades and the respective support ribs with a minimal loss ofenergy and is discharged to the outside of the fan 1. Furthermore, theair is allowed to smoothly pass through the vicinity of the respectiveblades and the respective support ribs, which makes it possible toefficiently rotate the respective impellers. As a consequence, it ispossible to reduce the amount of electric power consumed in therespective motors of the axial flow fans.

Furthermore, the end surfaces of the respective housings axiallycoincide with one another and the inner circumferential surfaces thereofare continuously joined to one another. This allows the air to smoothlyflow through the contra-rotating axial flow fan unit 1. As a result, itis difficult for the air to flow backwards, which makes it possible togreatly increase the volume of air and the static pressure. Due to thisfeature, it is possible to discharge the air present in a device casingand to supply a sufficient quantity of air to electronic parts in thedevice casing even when the system impedance within the device casing(namely, the flow path resistance within the device casing duringintroduction of the air) remains high.

The respective axial flow fans of the contra-rotating axial flow fanunit need not to be fixed to one another. FIG. 4 shows one modifiedexample of the contra-rotating axial flow fan unit in accordance with apreferred embodiment of the present invention. The respective axial flowfans are directly mounted to a casing of an electronic device or othersuitable apparatus to be axially spaced apart from one another, thusforming a contra-rotating axial flow fan unit 1 a. Referring to FIG. 4,there exist gaps between the intake side housing 23 and the intermediatehousing 43 and between the intermediate housing 43 and the exhaust sidehousing 33. It may also be possible to provide gaps between therespective housings by coupling the respective axial flow fans togetherwith spacers or other suitable gap-forming members or materialsinterposed therebetween.

With this construction, air is introduced through the gaps between therespective housings. This makes it possible to increase the volume ofthe air delivered from the contra-rotating axial flow fan unit 1 a.Furthermore, the contra-rotating axial flow fan unit 1 a can beconstructed by adjusting the gap size between the respective housings inconformity with the size of a casing of an electronic device or the likein which the contra-rotating axial flow fan unit 1 a is arranged. Thismakes it possible to apply the contra-rotating axial flow fan unit 1 ato device casings of different sizes.

The support ribs of the respective axial flow fans need not to bearranged on the exhaust side. FIG. 5 is a sectional view of anothermodified example of the contra-rotating axial flow fan unit inaccordance with a preferred embodiment of the present invention.

In this contra-rotating axial flow fan unit 1 b, the support ribs of therespective axial flow fans are arranged on the intake side. In thiscase, if the impellers of the respective axial flow fans are rotatinglydriven, the air is straightened by the intake side support ribs 24 andthen drawn into the intake side axial flow fan 2. This ensures that theair is smoothly drawn into the contra-rotating axial flow fan unit,thereby reducing the noise.

The inclination of the intake side support ribs 24 relative to thecentral rotation axis J1 is substantially the same as the inclination ofthe intake side blades 211 with respect to the central rotation axis J1.The air receiving surface 243 of each of the intake side support ribs 24is inclined with respect to the central rotation axis J1 so as to facetoward the exhaust side. The inclination of the intermediate supportribs 44 relative to the central rotation axis J1 is substantially thesame as the inclination of the intermediate blades 411 with respect tothe central rotation axis J1. The air receiving surface 443 of each ofthe intermediate support ribs 44 faces toward the exhaust side, thusconfronting the air stream generated by the rotation of the intake sideblades 211. Therefore, the air passes through the vicinity of therespective blades and the respective support ribs with a minimal loss ofenergy and is discharged to the outside of the contra-rotating axialflow fan unit 1 b. Furthermore, the air is allowed to smoothly passthrough the vicinity of the respective blades and the respective supportribs, which makes it possible to efficiently rotate the respectiveimpellers.

FIG. 6 is a perspective view showing a contra-rotating axial flow fanunit 1A in accordance with a second preferred embodiment of the presentinvention. FIG. 7 is a vertical sectional view of the contra-rotatingaxial flow fan unit 1A taken along a plane containing a central rotationaxis. In the following description, the same components of the fan 1A asthose of the contra-rotating axial flow fan unit 1 will be designated bylike reference numerals and description thereof will be omitted.

As shown in FIGS. 6 and 7, the contra-rotating axial flow fan unit 1A isa fourfold contra-rotating axial flow fan including an intake side axialflow fan 2, a first intermediate axial flow fan 4, a second intermediateaxial flow fan 5, and an exhaust side axial flow fan 3 arranged along acentral rotation axis J1 in the sequence described above. The mutuallyadjoining axial flow fans are fixed to one another by screws (notshown), engaging structures (not shown) or other suitable fixing membersor materials.

The intake side impeller 21 and the first intermediate impeller 41 arerotated about the central rotation axis J1 in different directions.Likewise, the first intermediate impeller 41 and the second intermediateimpeller 51 of the second intermediate axial flow fan 5 are rotatedabout the central rotation axis J1 in different directions. Furthermore,the second intermediate impeller 51 and the exhaust side impeller 31 ofthe exhaust side axial flow fan 3 are rotated about the central rotationaxis J1 in different directions.

In the present preferred embodiment, when seen from the axially upperside in FIG. 6, the intake side impeller 21 rotates clockwise, the firstintermediate impeller 41 rotates counterclockwise, the secondintermediate impeller 51 rotates clockwise, and the exhaust sideimpeller 31 rotates counterclockwise. Consequently, air is drawn intothe intake side axial flow fan 2 and delivered to the exhaust side axialflow fan 3. This generates an axially flowing air stream.

The second intermediate axial flow fan 5 includes a second intermediateimpeller 51, a second intermediate motor 52, a second intermediatehousing 53, and a plurality of second intermediate support ribs 54.

The second intermediate impeller 51 includes a plurality of (forexample, seven, in the present preferred embodiment) second intermediateblades 511 and a substantially cylindrical closed-top hub 512. On theouter surface of the hub 512, the second intermediate blades 511 extendradially outward about the central rotation axis J1 and arecircumferentially arranged at an equal pitch. The second intermediateblades 511 and the hub 512 are preferably integrally formed byinjection-molding a resin, for example. When seen from the upper side inFIG. 7, the second intermediate impeller 51 is rotated clockwise aboutthe central rotation axis J1 by the second intermediate motor 52. Thisgenerates an air stream flowing along the central rotation axis J1.

The second intermediate housing 53 is a hollow member preferably made ofa synthetic resin and has an inner circumferential surface ofsubstantially cylindrical shape designed to surround the secondintermediate impeller 51. Within the second intermediate housing 53(namely, on the inner circumferential surface of the second intermediatehousing 53), there is defined a flow path through which the air streamgenerated by the rotation of the second intermediate impeller 51 flows.The second intermediate housing 53 is provided with an upper end portionand a lower end portion each having an inner circumferential surfacearranged so that the distance between the central rotation axis J1 andthe inner circumferential surface can be increased axially upward ordownward. This ensures that the air is smoothly drawn into anddischarged from the second intermediate housing 53 as the secondintermediate blades 511 rotate.

The plurality of (for example, eight, in the present preferredembodiment) second intermediate support ribs 54 are preferably made of asynthetic resin. The second intermediate support ribs 54 are arrangedbelow the second intermediate impeller 51 (namely, between the secondintermediate impeller 51 and the exhaust side axial flow fan 3) andextend radially outward from the second intermediate motor 52 so thatthey can be connected to the second intermediate housing 53 to therebysupport the second intermediate motor 52.

As shown in FIG. 7, the second intermediate motor 52 includes a statorportion 521 and a rotor portion 522. The rotor portion 522 is supportedby a bearing mechanism so that it can rotate relative to the statorportion 521.

The stator portion 521 has a base portion 5211 that is substantiallydisk-shaped and arranged about the central rotation axis J1 when seen ina plan view. The base portion 5211 is fixed to the inner circumferentialsurface of the second intermediate housing 53 through the secondintermediate support ribs 54 to thereby hold the stator portion 521 inplace. The base portion 5211 is preferably made of a synthetic resin andis preferably integrally formed with the second intermediate supportribs 54 and the second intermediate housing 53 by injection-molding thesynthetic resin. However, the material and the method used in producingthe base portion 5211, the second intermediate support ribs 54, and thesecond intermediate housing 53 are not limited to synthetic resin andinjection molding. Alternatively, the base portion 5211, the secondintermediate support ribs 54, and the second intermediate housing 53 maybe formed by, e.g., die-casting an aluminum material or other suitablematerial.

As illustrated in FIG. 7, a substantially cylindrical bearing holderportion 5212 is fixed to the substantially central region of the baseportion 5211 so that it can protrude upward from the base portion 5211.The bearing holder portion 5212 is preferably made of a metal and ispreferably integrally fixed to the base portion 5211 byinjection-molding (e.g., insert-molding) with a resin. The bearingholder portion 5212 may be either formed from a synthetic resin,aluminum, aluminum alloy or other suitable materials or integrallyformed with the base portion 5211 and the second intermediate housing 53by injection-molding a resin or die-casting aluminum. The material andthe method used in producing the bearing holder portion 5212 are notparticularly limited. Ball bearings 5213 and 5214 are provided in theaxial upper and lower regions inside the bearing holder portion 5212,thus defining a portion of the bearing mechanism. An elastic member(e.g., a spring) is preferably arranged to pre-compress the ball bearing5214 from above.

The stator portion 521 is provided with a stator 5215 and a circuitboard 5216. The stator 5215 is attached to the outer surface of thebearing holder portion 5212. The circuit board 5216 preferably has asubstantially annular flat shape and is attached to the lower side ofthe stator 5215. The circuit board 5216 is provided with a circuitarranged to control the rotation of the rotor portion 522 and iselectrically connected to the stator 5215 through a conductive pin andthe like. An electric current and a control signal are sent from anexternal power source (not shown) to the circuit board 5216 via a leadline group (not shown) having a plurality of lead lines tied together.

The rotor portion 522 includes a yoke 5221, a field magnet 5222, and ashaft 5223.

The yoke 5221 has a substantially cylindrical closed-top shape and ismade of a magnetic metal. The field magnet 5222 preferably has asubstantially cylindrical shape and has an outer circumferential surfacefixed to the inner surface of the cylindrical portion of the yoke 5221by an adhesive agent or suitable fixing materials or members. The fieldmagnet 5222 has an inner circumferential surface radially opposite tothe stator 5215. An axially protruding cylinder portion is provided inthe substantially central region of the cover portion of the yoke 5221and may be formed by burring or other suitable process. The shaft 5223is press-fitted into the cylinder portion in a coaxial relationship withthe central rotation axis J1.

The shaft 5223 is inserted into the bearing holder portion 5212 and issupported by the ball bearings 5213 and 5214 for rotation relative tothe stator portion 521. That is, in the second intermediate axial flowfan 5, the shaft 5223 and the ball bearings 5213 and 5214 define abearing mechanism arranged to support the yoke 5221 so that it canrotate about the central rotation axis J1 with respect to the baseportion 5211.

As a driving current is supplied from the external power source to thestator 5215 through the lead line group and the circuit board 5216, thetorque acting about the central rotation axis J1 is generated betweenthe stator 5215 and the field magnet 5222. The rotor portion 522 and thesecond intermediate impeller 51 are rotated by the torque thusgenerated. The driving current supplied to the stator 5215 is controlledby the circuit in the circuit board 5216. This makes it possible torotate the second intermediate impeller 51 at a predetermined rotationspeed.

The upper and lower end surfaces of the first intermediate housing 43axially coincide with the lower end surface of the intake side housing23 and the upper end surface of the second intermediate housing 53,respectively. Furthermore, the upper and lower end surfaces of thesecond intermediate housing 53 axially coincide with the lower endsurface of the first intermediate housing 43 and the upper end surfaceof the exhaust side housing 33, respectively.

When observing the contra-rotating axial flow fan unit 1A as a whole,the respective flow paths (i.e., the respective inner circumferentialsurfaces) of the intake side housing 23, the first intermediate housing43, the second intermediate housing 53, and the exhaust side housing 33are successively and smoothly connected to one another along the axialdirection, thereby defining a single flow path (i.e., a single innercircumferential surface). The intake side impeller 21, the intake sidesupport ribs 24, the first intermediate impeller 41, the firstintermediate support ribs 44, the second intermediate impeller 51, thesecond intermediate support ribs 54, the exhaust side impeller 31, andthe exhaust side support ribs 34 are arranged in the single flow pathone below another from the upper side (i.e., the intake side) in FIG. 7.

In the contra-rotating axial flow fan unit 1A, the axially adjacentblades and support ribs differ in number from one another. Specifically,the number of the intake side support ribs 24 differs from the number ofthe blades (the intake side blades 211 and the first intermediate blades411) of the impellers axially adjacent thereto (i.e., the intake sideimpeller 21 and the first intermediate impeller 41). Furthermore, thenumber of the first intermediate blades 411 differs from the number ofthe support ribs axially adjacent thereto (i.e., the intake side supportribs 24 and the first intermediate support ribs 44). Moreover, thenumber of the first intermediate support ribs 44 differs from the numberof the blades (i.e., the first intermediate blades 411 and the secondintermediate blades 511) of the impellers axially adjacent thereto(i.e., the first intermediate impeller 41 and the second intermediateimpeller 51). In addition, the number of the second intermediate blades511 differs from the number of the support ribs axially adjacent thereto(i.e., the first intermediate support ribs 44 and the secondintermediate support ribs 54). The number of the second intermediatesupport ribs 54 differs from the number of the blades (i.e., the secondintermediate blades 511 and the exhaust side blades 311) of theimpellers axially adjacent thereto (i.e., the second intermediateimpeller 51 and the exhaust side impeller 31). The number of the exhaustside blades 311 differs from the number of the exhaust side support ribs34.

The air stream generated by the rotation of the respective bladesimpinges on the respective support ribs axially adjacent to the blades,thus generating a noise. If the respective blades and the respectivesupport blades adjacent to each other along the central rotation axis J1differ in number from each other, the frequency of a wind noisegenerated by the rotation of the respective impellers does not coincidewith the frequency of an interfering noise generated when the air streamimpinges against the respective support ribs. Therefore, it is possibleto prevent an increase in noise which would otherwise occur by thesympathetic vibration of the wind noise and the interfering noise. Thismakes it possible to reduce the noise generated from the contra-rotatingaxial flow fan unit 1A.

As described above, a single flow path defined by the innercircumferential surfaces of the respective housings successivelyconnected to one another in the axial direction is formed within thecontra-rotating axial flow fan unit 1A. The air stream introduced fromthe intake side axial flow fan 2 is allowed to smoothly flow along thesingle flow path (i.e., the inner circumferential surfaces of therespective housings). Then, the air stream is discharged from theexhaust side axial flow fan 3 to the outside with a minimal loss ofenergy due to frictional contact of the air stream with the flow path.

FIG. 8 is a sectional view illustrating only the cross-sections of theintake side blades 211, the intake side support ribs 24, the firstintermediate blades 411, the first intermediate support ribs 44, thesecond intermediate blades 511, the second intermediate support ribs 54,the exhaust side blades 311, and the exhaust side support ribs 34 takenalong a cylindrical plane having an arbitrary radius measured from thecentral rotation axis J1 in FIG. 7.

Each of the intake side support ribs 24 is substantially flat and has anupper edge 241 positioned adjacent to each of the intake side blades 211and a lower edge 242 positioned adjacent to the first intermediate axialflow fan 4. The upper edge 241 is positioned upstream of the lower edge242 with reference to the rotational direction R2. Each of the intakeside support ribs 24 has an air receiving surface 243 inclined relativeto the central rotation axis J1 to face toward the exhaust side. Thus,the air receiving surface 243 confronts the air stream generated by therotation of the intake side blades 211.

The air stream generated by the rotation of the intake side impeller 21passes through the vicinity of the air receiving surface 243 and isintroduced toward each of the first intermediate blades 411. Each of thefirst intermediate blades 411 is inclined with respect to the centralrotation axis J1 substantially in the same direction as is each of theintake side support ribs 24. Therefore, the air stream whose velocitycomponent has been changed by the intake side support ribs 24 issmoothly introduced toward the first intermediate blades 411, whichmakes it possible to minimize the reduction of energy of the air stream.

Each of the first intermediate support ribs 44 is substantially flat andhas an upper edge 441 positioned adjacent to each of the firstintermediate blades 411 and a lower edge 442 positioned adjacent to thesecond intermediate axial flow fan 5. The upper edge 441 is positionedupstream of the lower edge 442 with reference to the rotationaldirection R4. Each of the first intermediate support ribs 44 has an airreceiving surface 443 inclined relative to the central rotation axis J1to face toward the exhaust side. Thus, the air receiving surface 443confronts the air stream generated by the rotation of the firstintermediate blades 411.

The swirling-direction velocity component generated by the rotation ofthe first intermediate blades 411 acts substantially in the samedirection as the rotational direction R4. A portion of theswirling-direction velocity component impinges against each of the firstintermediate support ribs 44 and, therefore, is changed to a velocitycomponent acting in the same direction as the central rotation axis J1.This increases the static pressure of the air.

The air stream passed through the vicinity of the air receiving surface443 is introduced toward each of the second intermediate blades 511. Asshown in FIG. 8, each of the second intermediate blades 511 is inclinedwith respect to the central rotation axis J1 substantially in the samedirection as is each of the first intermediate support ribs 44.Therefore, the air stream whose velocity component has been changed bythe first intermediate support ribs 44 is smoothly introduced toward thesecond intermediate blades 511, which makes it possible to reduce theloss of energy of the air stream.

Each of the second intermediate support ribs 54 has an upper edge 541positioned adjacent to each of the second intermediate blades 511 and alower edge 542 positioned adjacent to the exhaust side axial flow fan 3.The upper edge 541 is positioned upstream of the lower edge 542 withreference to the rotational direction R5. Each of the secondintermediate support ribs 54 has an air receiving surface 543 inclinedrelative to the central rotation axis J1 to face toward the exhaustside. Thus, the air receiving surface 543 confronts the air streamgenerated by the rotation of the second intermediate blades 511.

The swirling-direction velocity component generated by the rotation ofthe second intermediate blades 511 acts substantially in the samedirection as the rotational direction R5. A portion of theswirling-direction velocity component impinges against each of thesecond intermediate support ribs 54 and, therefore, is changed to avelocity component acting in the same direction as the central rotationaxis J1. This increases the static pressure of the air.

The air stream passed through the vicinity of the air receiving surface543 is introduced toward each of the exhaust side blades 311. As shownin FIG. 8, each of the exhaust side blades 311 is inclined with respectto the central rotation axis J1 substantially in the same direction asis each of the second intermediate support ribs 54. Therefore, the airstream whose velocity component has been changed by the secondintermediate support ribs 54 is smoothly introduced toward the exhaustside blades 311, which makes it possible to reduce the loss of energy ofthe air stream.

The swirling-direction velocity component generated by the rotation ofthe exhaust side blades 311 acts substantially in the same direction asthe rotational direction R3. A portion of the swirling-directionvelocity component impinges against each of the exhaust side supportribs 34 and, therefore, is changed to a velocity component acting in thesame direction as the central rotation axis J1. This increases thestatic pressure of the air.

Each of the exhaust side support ribs 34 has an upper edge 341positioned adjacent to each of the exhaust side blades 311 and a loweredge 342 positioned adjacent to the exhaust side. The upper edge 341 ispositioned upstream of the lower edge 342 with reference to therotational direction R3. Each of the exhaust side support ribs 34 has anair receiving surface 343 inclined relative to the central rotation axisJ1 to face toward the exhaust side. Thus, the air receiving surface 343confronts the air stream generated by the rotation of the exhaust sideblades 311.

The air stream passed through the vicinity of the air receiving surface343 is discharged to the outside of the exhaust side housing 33 (namely,the outside of the contra-rotating axial flow fan unit 1A).

The construction described above ensures that the air drawn into thecontra-rotating axial flow fan unit 1A passes through the vicinity ofthe respective blades and the respective support ribs with a minimalloss of energy and is discharged to the outside of the fan 1A.Furthermore, the air is allowed to smoothly pass through the vicinity ofthe respective blades and the respective support ribs, which makes itpossible to efficiently rotate the respective impellers. As aconsequence, it is possible to reduce the amount of electric powerconsumed in the respective motors of the axial flow fans.

Furthermore, the end surfaces of the respective housings axiallycoincide with one another and the inner circumferential surfaces thereofare continuously joined to one another. This allows the air to smoothlyflow through the contra-rotating axial flow fan unit 1A. As a result, itbecomes difficult for the air to flow backwards, which makes it possibleto greatly increase the volume of air and the static pressure. Due tothis feature, it is possible to discharge the air present in a devicecasing and to supply a sufficient quantity of air to electronic parts inthe device casing, even when the system impedance within the devicecasing (namely, the flow path resistance within the device casing duringintroduction of the air) remains high.

As in the modified examples described above, the respective axial flowfans may be spaced apart along the central rotation axis J1 or spacersmay be arranged between the respective axial flow fans in the presentpreferred embodiment. The support ribs of the respective axial flow fansmay be arranged on the intake side along the axial direction. In thiscase, the air receiving surface of each support rib of the intermediateaxial flow fans and the exhaust side axial flow fan is oriented to facetoward the exhaust side. The inclination of the air receiving surfacerelative to the central rotation axis J1 is substantially the same asthe inclination of each blade of the respective exhaust side impellerswith respect to the central rotation axis J1. The inclination of theintake side support ribs 24 is substantially the same as the inclinationof the intake side blades 211 with respect to the central rotation axisJ1. This construction ensures that the air is smoothly drawn into thecontra-rotating axial flow fan unit, thereby reducing a noise.

The preferred embodiments of the contra-rotating axial flow fan unitsdescribed above are merely non-limiting examples. The contra-rotatingaxial flow fan units are not limited to the aforementioned shapes andconfigurations insofar as the impellers of the adjoining axial flow fansdiffer from one another in their rotational direction.

The number of the axial flow fans used in the contra-rotating axial flowfan units is not limited and may be, e.g., five or more, insofar as therespective axial flow fans adjoining to one other along the centralrotation axis J1 differ in their rotational direction.

The cross-sectional shape of the respective support ribs is notparticularly limited but may be a substantially circular shape, asubstantially elliptical shape, a substantially polygonal shape or othershapes. It is preferred that the respective support ribs are shaped tohave an upper edge, a lower edge, and an air receiving surface.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. An axial flow fan unit comprising: an intake side axial flow fanincluding: an intake side impeller arranged to rotate about a centralrotation axis and having intake side blades; an intake side motorarranged to rotate the intake side impeller; and an intake side housinghaving an inner circumferential surface that surrounds the intake sideimpeller; an exhaust side axial flow fan including: an exhaust sideimpeller arranged to rotate about the central rotation axis and havingexhaust side blades; an exhaust side motor arranged to rotate theexhaust side impeller; and an exhaust side housing having an innercircumferential surface that surrounds the exhaust side impeller; and atleast one intermediate axial flow fan arranged between the intake sideaxial flow fan and the exhaust side axial flow fan, each of the at leastone intermediate axial flow fan including: an intermediate impellerarranged to rotate about the central rotation axis and havingintermediate blades; an intermediate motor arranged to rotate theintermediate impeller; and an intermediate housing having an innercircumferential surface that surrounds the intermediate impeller;wherein a rotational direction of the intermediate impeller is differentfrom that of the impellers adjacent thereto.
 2. The axial flow fan unitof claim 1, wherein the intake side axial flow fan includes intake sidesupport ribs arranged to interconnect the intake side motor and theintake side housing; the exhaust side axial flow fan includes exhaustside support ribs arranged to interconnect the exhaust side motor andthe exhaust side housing; and the intermediate axial flow fan includesintermediate support ribs arranged to interconnect the intermediatemotor and the intermediate housing.
 3. The axial flow fan unit of claim2, wherein the intake side support ribs are arranged between the intakeside impeller and the intermediate impeller, and the intermediatesupport ribs are arranged between the intermediate impeller and theexhaust side impeller.
 4. The axial flow fan unit of claim 3, whereinthe number of the blades in each axial flow fan is different from thenumber of each of the support ribs axially adjacent to the blades. 5.The axial flow fan unit of claim 1, wherein the intermediate housing issuccessively arranged with the intake side housing and the exhaust sidehousing in an axial direction.
 6. The axial flow fan unit of claim 5,wherein the intermediate housing is successively arranged with theintake side housing and the exhaust side housing in the axial directionwith gaps arranged between each of the housings in the axial direction.7. The axial flow fan unit of claim 2, wherein the intake side supportribs, the exhaust side support ribs, and the intermediate support ribsare substantially flat.
 8. The axial flow fan unit of claim 2, whereineach of the intake side support ribs, the exhaust side support ribs, andthe intermediate support ribs has an upper edge and a lower edge.
 9. Theaxial flow fan unit of claim 8, wherein each of the intake side supportribs, the exhaust side support ribs, and the intermediate support ribshas an air receiving surface arranged axially between the upper edge andthe lower edge to confront an air stream flowing from the intake sideimpeller toward the exhaust side impeller.
 10. The axial flow fan unitof claim 9, wherein the air receiving surface axially faces an exhaustside of the respective axial fan.
 11. The axial flow fan unit of claim2, wherein each of the intake side support ribs, the exhaust sidesupport ribs, and the intermediate support ribs is inclined with respectto the central rotation axis.
 12. The axial flow fan unit of claim 11,wherein each of the intake side blades, the exhaust side blades, and theintermediate blades is inclined with respect to the central rotationaxis, the inclination of each of the blades being substantially the sameas the inclination of each of the support ribs axially positioned at anintake side of the blades.
 13. The axial flow fan unit of claim 8,wherein the upper edge of each of the intake side support ribs ispositioned upstream of the lower edge thereof with reference to arotational direction of the intake side impeller; the upper edge of eachof the exhaust side support ribs is positioned upstream of the loweredge thereof with reference to a rotational direction of the exhaustside impeller; and the upper edge of each of the intermediate supportribs is positioned upstream of the lower edge thereof with reference toa rotational direction of the intermediate impeller.
 14. The axial flowfan unit of claim 2, wherein the intake side support ribs are arrangedat an intake side of the intake side housing; the intermediate supportribs are arranged between the intake side impeller and the intermediateimpeller; and the exhaust side support ribs are arranged between theintermediate impeller and the exhaust side impeller.
 15. The axial flowfan unit of claim 1, wherein the at least one intermediate axial flowfan includes at least a first intermediate axial flow fan and a secondintermediate axial flow fan.
 16. The axial flow fan unit of claim 15,wherein the intake side axial flow fan includes intake side support ribsarranged to interconnect the intake side motor and the intake sidehousing; the exhaust side axial flow fan includes exhaust side supportribs arranged to interconnect the exhaust side motor and the exhaustside housing; and each of the intermediate axial flow fans includesintermediate support ribs arranged to interconnect the intermediatemotor and the intermediate housing.
 17. The axial flow fan unit of claim16, wherein the intake side support ribs are arranged between the intakeside impeller and the intermediate impeller; the intermediate supportribs in the first intermediate axial flow fan are arranged between theintermediate impeller in the first intermediate axial flow fan and theintermediate support ribs in the second intermediate axial flow fan; andthe exhaust side support ribs are opposite to the exhaust side impelleraway from the intermediate impeller in the second intermediate axialflow fan.
 18. The axial flow fan unit of claim 17, wherein the number ofthe blades in each axial flow fan is different from the number of eachof the support ribs axially adjacent to the blades.
 19. The axial flowfan unit of claim 15, wherein the intermediate housing of the firstintermediate axial flow fan is axially opposite to the intake sidehousing, the exhaust side housing, or the second intermediate housing.20. The axial flow fan unit of claim 15, wherein the intermediatehousing of the first intermediate axial flow fan is opposite to theintake side housing, the exhaust side housing, or the secondintermediate housing with gaps arranged between each of the housings inthe axial direction.
 21. The axial flow fan unit of claim 16, whereinthe intake side support ribs are arranged at an intake side of theintake side housing; the intermediate support ribs in the firstintermediate axial flow fan are arranged between the intake sideimpeller and the intermediate impeller in the first intermediate axialflow fan, or between the intermediate impeller in the first intermediateaxial flow fan and the intermediate impeller in the second intermediateaxial flow fan; and the exhaust side support ribs are arranged betweenthe exhaust side impeller and the intermediate impeller in the secondintermediate axial flow fan.